This invention relates in general to telephone communication circuitry and, in particular, to suppressing echo returned from one end of a four-wire transmission path due to the reflection of signals originating at the other end. The source of such echo is often an impedance mismatch at the hybrid circuit, which provides conversion from four-wire to two-wire transmission, at the end of the four-wire section.
Echo suppressors and their more sophisticated improvements, echo cancellers, are, of course, well known and widely used in long distance transmission. Existing echo suppressors make their decisions on the basis of the short-term average power level of two signals. This type of suppressor is relatively expensive and difficult to implement, particularly in digital form. The invention here uses peak signal level and a simple threshold memory, rather than average power, which yields much simpler, less-expensive implementation.
Echo suppression is normally necessary only on circuits having a long delay. In the past long delay could be equated to long distance. On relatively expensive long distance circuits the expense of a high-cost echo suppressor or echo canceller could be tolerated. With the advent of digital local transmission, and even voice transmission on local area networks originally designed for data, long delays requiring echo treatment can be generated even on low-cost local circuits where economical echo suppression is more important. A particular problem exists for telephone communication on a local area network which uses token passing or carrier sense multiple access techniques, and which therefore provides relatively long, sometimes variable delay times between burst transmissions. Virtual voice circuits derived by means of local area networks require both speed changing buffers and additional buffering to smooth out variable delays. In order to keep overhead requirements reasonable, packets, which are relatively long in voice terms, are used. For reasonable size buffers absolute delays of several tens of milliseconds will occur even on local connections.
Local area networks are themselves typically four-wire, and echo-might not occur; however, a practical local area network for voice must interface two-wire telephone circuits. Echo will occur at the two-wire to four-wire connection point. Therefore, echo control must be incorporated since the subjective effect of echo is increased by delay anywhere in the echo path and the buffer used in such networks produce delays sufficiently long to be troublesome.
The present invention provides a novel digital circuit for echo suppression which is particularly simple to implement in digital form compared with previous techniques. One embodiment of the present invention is a "half echo suppressor" or "split echo suppressor," which suppresses echo from one direction only. In the local area network application mentioned above, echo occurs only in one direction. In fact, in most applications, even if full, both-ended echo suppression is required, it is accomplished with two such devices, one at each end of the circuit, rather than a single device in the middle. An extension to a full echo suppressor is another embodiment of the present invention. Related to this embodiment the term "four-wire transmission" is, of course, used in the generic sense to denote a circuit having a separate path for each of the two directions of conversation whether derived on four conductors, three conductors, or by time or frequency division multiplexing or by other means, and whether or not he representation is analog or digital.